a) Field of the Invention
The invention relates to an ultrasonic transducer for lithotripsy in which piezoelectric transducer elements are fixed to a backing and are connected on their front side to first electrodes and their rear side to second electrodes which can be connected to an electric pulse generator to produce electric fields by means of pulses in the transducer elements via the electrodes and to oscillate the transducer elements.
b) Description of the Prior Art
Ultrasonic transducers designed in this way are known. The way in which they work is based on producing a mechanical state of stress suddenly in the interior of the piezoelectric transducer element when a voltage is applied, which mechanical state of stress is produced by setting up an electric field in the interior of the transducer element which has the effect of each layer of the transducer element trying to change its thickness. When the electrodes of the ceramic body in the transducer elements are plane parallel, a homogeneous, electric field is thus produced in the interior thereof which also produces pressure pulses of negative sign in addition to positive pulses in transducer elements fixed to a reflection-free rear-side backing. These occur as pulse trains which bring with them the danger of tissue damage in the focal area, especially with the conventional focusing transducers.
Various measures which have the aim of reducing the amplitude of the negative pulse waves emitted, have been taken to minimise this danger.
Hence a piezoelectric transducer can be seen from German Patentschrift 3 425 992 in which a number of transducer elements are arranged on the front side of a spherical cup-shaped backing. The cup material is thus selected so that its wave resistance hardly differs from that of the ceramic elements and the rear side of the surface of the cup is shaped so that the sound waves produced by the ceramic elements and reflected on the cup are not focused. Hence a favourable ratio of the amplitudes of the positive and negative pulses can indeed be achieved. However, further required reduction of the negative pulses cannot be achieved as their shape is predetermined at the border layer transducer element-backing by the existing geometry of the transducer elements and the arrangement of the electrodes.
German Offenlegungsschrift 3 119 295 shows a further device for destroying concrements in which the danger of tissue damage is reduced by the ultrasonic transducer to be focused on the concrement having such a large surface area that, on the one hand the sound intensity is low on the transmission path, but on the other hand is so large at the focus that it is sufficient to destroy the concrement situated at the focus.
It is therefore the main object of the invention to provide an ultrasound tranducer having a high sound intensity at the focus and in which the negative pressure pulse is reduced so far that any danger of tissue damage in the area of the concrement and on the transmission path is avoided.